Improved catalyst for mwcnt production

ABSTRACT

An iron-free supported catalyst for the selective conversion of hydrocarbons to carbon nanotubes may include cobalt and vanadium as active catalytic metals in any oxidation state on a catalyst support comprising aluminum oxide hydroxide. The mass ratio of cobalt to vanadium is between 2 and 15; the mass ratio of cobalt to aluminum is between 5.8 10−2 and 5.8 10−1; and the mass ratio vanadium to aluminum is between 5.8 10−3 and 8.7 10−2. The present disclosure is further related to a method for the production of this iron-free supported catalyst and to a method for the production of carbon nanotubes using the iron-free supported catalyst.

FIELD

The present disclosure relates to a supported catalyst system, for theconversion of hydrocarbons into carbon nanotubes and in particular to aniron-free supported catalyst system for a multi-walled carbon nanotubeproduction process with improved selectivity and yield.

INTRODUCTION

Carbon nanostructures (CNSs) refer collectively to nanosized carbonstructures having various shapes, such as nanotubes, nanohairs,fullerenes, nanocones, nanohorns, and nanorods. Carbon nanostructurescan be widely utilized in a variety of technological applicationsbecause they possess excellent characteristics.

Carbon nanotubes (CNTs) are tubular materials consisting of carbon atomsarranged in a hexagonal pattern and have a diameter of approximately 1to 100 nm. Carbon nanotubes exhibit insulating, conducting orsemi-conducting properties depending on their inherent chirality. Carbonnanotubes have a structure in which carbon atoms are strongly covalentlybonded to each other. Due to this structure, carbon nanotubes have atensile strength approximately 100 times greater than that of steel,they are highly flexible and elastic, and are chemically stable

Carbon nanotubes are divided into three types: single-walled carbonnanotubes (SWCNTs) consisting of a single sheet and having a diameter ofabout 1 nm; double-walled carbon nanotubes (DWCNTs) consisting of twosheets and having a diameter of about 1.4 to about 3 nm; andmulti-walled carbon nanotubes (MWCNTs) consisting of three or moresheets and having a diameter of about 5 to about 100 nm.

Carbon nanotubes are being investigated for their commercialization andapplication in various industrial fields, for example, aerospace, fuelcell, composite material, biotechnology, pharmaceutical,electrical/electronic, and semiconductor industries, due to their highchemical stability, flexibility and elasticity.

Carbon nanotubes are generally produced by various techniques, such asarc discharge, laser ablation, and chemical vapor deposition. However,arc discharge and laser ablation are not appropriate for mass productionof carbon nanotubes and require high arc production costs or expensivelaser equipment. Catalytic Chemical Vapor Deposition (CCVD) ofhydrocarbons over metallic catalysts provides, with respect to othermethods, higher yields and quality and simplifies the manufacturingprocess on an industrial scale.

Most researches carried out in the CCVD technology are presently focusedon developing new catalysts and reaction conditions for controlling thetype (single, double or multi-walled), diameter, length and purity ofcarbon nanotubes. The structural, physical and chemical properties ofcarbon nanotubes have been related to their electrical conductingcapacity, mechanical strength and thermal, optical and magneticproperties.

WO 03/004410 A1 discloses a large variety of metal oxide systems (suchas Co, Fe, Ni, V, Mo and Cu) and catalyst supports (such as Al(OH)₃,Ca(OH)₂, Mg(OH)₂, Ti(OH)₄, Ce(OH)₄ and La(OH)₃), for the single-walledand multi-walled carbon nanotube production. The various metals andmixtures of metals in this document were tested for their selectivityproperties, i.e. the ability of the catalyst to selectively producesingle, double or multi-walled nanotubes with respect to a certainproportion of amorphous carbon or fibers formed simultaneously duringthe reaction.

EP 2 883 609 A1 discloses an impregnated supported catalyst and a carbonnanotube aggregate comprising the impregnated supported catalyst, saidimpregnated supported catalyst being prepared by sequentially adding amulti-carboxylic acid and precursors of first (Co) and second (Fe, Ni)catalytic components to precursors of first (Mo) and second (V) activecomponents to obtain a transparent aqueous metal solution, impregnatingan aluminum-based granular support with the transparent aqueous metalsolution, followed by drying and calcination, wherein the supportedcatalyst has a bulk density of 0.8 to 1.5 g/cm³.

U.S. Pat. No. 9,956,546 A1 discloses a catalyst for producing carbonnanotubes, comprising a support and a graphitization metal catalystsupported on the support wherein the graphitization metal catalyst is amulti-component metal catalyst comprising a main catalyst and anauxiliary catalyst, wherein the main catalyst is selected from Co, Fe,and mixtures thereof and the auxiliary catalyst is V, and wherein thecatalyst is a supported catalyst obtained by calcining aluminumhydroxide at a primary calcination temperature of 250° C. to 500° C. toform the support, supporting a catalytic metal precursor on the support,and calcining the catalytic metal precursor supported on the support ata secondary calcination temperature of 450° C. to 800° C.

EP 3 053 877 A1 discloses a method for producing carbon nanotubes,comprising primarily calcining support precursor having a BET specificsurface area of 1 m²/g or less at a temperature of 100 to 450° C. toform a support, supporting a graphitization metal catalyst on thesupport, secondarily calcining the catalyst supported on the support ata temperature of 100 to 500° C. to prepare the supported catalyst, andbringing the supported catalyst into contact with a carbon source in thegas phase to form carbon nanotubes, wherein the support precursor isaluminum trihydroxide and wherein the graphitization metal catalyst is abinary metal catalyst selected from Co/Mo, Co/V, Fe/Mo and Fe/V.

US 2008 213160 A1 discloses a method for synthesizing a supportedcatalyst with a view to the production of multi-walled carbon nanotubescomprising the following steps: mixing an Al(OH)₃ powder having aparticle size lower than about 80 μm with an aqueous solution of an ironand cobalt salt, the whole forming a paste; drying said paste until apowder with a moisture level lower than about 5% by weight is obtained;selecting the particle-size fraction of said supported catalyst that islower than about 63 μm; and producing nanotubes using the supportedcatalyst having a particle size lower than about 63 μm.

KR 101781252 discloses a method for producing carbon nanotube aggregatescomprising heat treating a carrier precursor comprising a layered metalhydroxide and a non-layered metal hydroxide to form a porous carrier;supporting a catalyst metal or a catalyst metal precursor on the carrierto form a supported catalyst; and forming a carbon nanotube aggregate inwhich the supported catalyst and the carbon-containing compound arebrought into contact with each other under a heating region to form abundle and an entangled carbon nanotube aggregate. The catalyst metalcombines an element selected from iron, cobalt, and nickel, an elementselected from titanium, vanadium, and chromium, and an element selectedfrom molybdenum (Mo) and tungsten (W).

EP 3 156 125 A1 discloses a method for producing a carbon nanotubeaggregate, comprising:

-   -   mixing a support with an aqueous solution of a graphitization        metal catalyst precursor to form a paste;    -   drying the paste to remove water, followed by calcination to        obtain a supported catalyst; and    -   bringing the supported catalyst into contact with a        carbon-containing compound under heating to react with each        other,    -   wherein the water removal rate from the paste is adjusted to 5        to 30% by weight to control the bulk density of the carbon        nanotubes.        The graphitization catalyst is a catalyst containing only iron        (Fe) or a binary or multi-component catalyst comprising one or        more metals selected from cobalt (Co), molybdenum (Mo), and        vanadium (V).

EP 3 053 880 A1 discloses a method for producing a carbon nanotubeaggregate, comprising calcining aluminum hydroxide at a primarycalcination temperature of 100° C. to 500° C. to form a support;supporting a catalytic metal precursor on the support; calcining thecatalyst-containing support at a secondary calcination temperature of100° C. to 800° C. to obtain a supported catalyst; and bringing thesupported catalyst into contact with a carbon-containing compound underheating to react with each other, wherein the primary calcinationtemperature, the secondary calcination temperature, the amount of thecatalyst supported or the reaction time is controlled such that thecarbon nanotube aggregate has a bulk density of 10 kg/m³ or more. Thecatalytic metal comprises Fe, Co, Mo, V or a combination of two or morethereof. The graphitization metal catalyst may be a composite catalystconsisting of a main catalyst and an auxiliary catalyst. In this case,the main catalyst may include iron (Fe) or cobalt (Co) and the auxiliarycatalyst may be molybdenum (Mo), vanadium (V) or a combination thereof.An organic acid is added in a molar ratio of 5:1 to 30:1 relative to thecatalytic metal for the preparation of the supported catalyst.

Carbon nanotubes have attracted attention as potential electrodematerials in lithium batteries.

Typical lithium-ion batteries utilize carbon anodes (negative electrode)and lithiated transition metal oxide cathodes (positive electrode)situated on opposite sides of a microporous polymer separator.

A lithium-ion cell begins life with all of the lithium in the cathodeand upon charging, a percentage of this lithium is moved over to theanode and intercalated within the carbon anode.

A failure in lithium-ion batteries is the result of a formation ofdendrites within the battery. Dendrites are microscopic metal depositsthat can form within the cell. Dendrite formation generally begins inthe anode and creates an internal shortcut when it extends through theseparator to the cathode.

When iron impurities from any electrode dissolve in the electrolyte,there is a significant risk that these impurities migrate on the anodeside and initiate dendrite growth by deposition. Because of this,iron-free materials are required as electrode material.

When using MWCNT's as electrode material, the risk of battery failurecaused by those dendrites arises.

Consequently, MWCNT's comprising interstitial iron-components obtainedby a process using a catalytic system comprising an iron-basedgraphitization catalyst should be avoided.

Therefore, a need exists for MWCNT, produced by a CCVD-process ofhydrocarbons over iron-free metallic catalysts with improved selectivityand productivity.

SUMMARY

The aim of the present disclosure is to disclose an iron-free catalystfor the preparation of MWCNT and a method for its preparation, as wellas the use of those carbon nanotubes in batteries.

The present disclosure describes an iron-free supported catalyst for theselective conversion of hydrocarbons to carbon nanotubes, said catalystcomprising cobalt and vanadium as active catalytic metals in anyoxidation state on a catalyst support comprising aluminum oxidehydroxide wherein:

-   -   the mass ratio of cobalt to vanadium is between 2 and 15;    -   the mass ratio of cobalt to aluminum is between 5.8 10⁻² and 5.8        10⁻¹; and    -   the mass ratio of vanadium to aluminum is between 5.8 10⁻³ and        8.7 10⁻².

Preferred embodiments disclose one or more of the following features:

-   -   the mass ratio of cobalt to vanadium is between 3.0 and 11;    -   the mass ratio of cobalt to aluminum is between 1.2 10⁻¹ and 4.3        10⁻¹; and    -   the mass ratio of vanadium to aluminum is between 1.2 10⁻² and        5.8 10⁻²;

-   the iron-free catalyst of the present disclosure additionally    comprises molybdenum as active catalyst, wherein:    -   the mass ratio of molybdenum to aluminum is between 1.2 10⁻³ and        2.3 10⁻²; and    -   the mass ratio of cobalt to the combined mass of vanadium and        molybdenum is between 2 and 15;

-   the iron-free molybdenum comprising the catalyst of the present    disclosure wherein:    -   the mass ratio of molybdenum to aluminum is between 1.7 10⁻³ and        1.7 10⁻²; and    -   the mass ratio of cobalt to the combined mass of vanadium and        molybdenum is between 3 and 11;

-   the catalyst support of the present disclosure comprises at least    30% by weight of aluminum oxide hydroxide, based on the total of    aluminum hydroxides and/or aluminum oxides and aluminum oxide    hydroxide;

-   the iron-free catalyst has a maximum diffraction peak at a 2θ angle    of 35° to 38° in the XRD pattern recorded in the 2θ range of 10° to    80°, wherein    -   when the intensity of the maximum diffraction peak and the        intensity of a diffraction peak at a 2θ angle of 17° to 22° are        defined as “a” and “b”, respectively, the ratio b/a is in the        range of 0.10 to 0.7, and    -   when the intensity of a diffraction peak at a 2θ angle of 63° to        67° is defined as “c”, the ratio c/a is in a range of 0.51 to        0.7

The present disclosure further discloses a method for the production ofthe iron-free supported catalyst comprising the steps of:

-   -   contacting an aqueous solution, comprising one or more        polycarboxylic acid(s) and/or the salt(s) of polycarboxylic        acids, with one or more vanadium-based precursor(s) and        optionally one or more molybdenum-based precursor(s);    -   contacting the one or more cobalt-based precursor(s) with the        aqueous solution comprising the vanadium-based precursor(s) and        the optional additional molybdenum-based precursor(s) to form a        water-based mixture of catalytic precursors;    -   contacting aluminum hydroxide, having a BET comprised between 3        and 18 m²/g, with the water-based mixture comprising the        catalytic precursors to form a water-based mixture of aluminum        hydroxide and the catalytic precursors;    -   drying the water-based mixture of aluminum hydroxide and the        catalytic precursors to form a dried mixture;    -   calcinating the dried mixture at a temperature of at least 200°        C., to form a calcinated product;    -   grinding the calcinated product to a powder.

Preferred embodiments of the method for the production of iron-freesupported catalyst disclose one or more of the following features:

-   -   the water-based mixture of aluminum hydroxide and the catalytic        precursors is dried at a predetermined temperature of at least        100° C. for at least 1 hour with an air flow of at least 0.1        m³/h;    -   the water-based mixture of aluminum hydroxide and the catalytic        precursors is dried at a predetermined temperature comprised        between 100 and 150° C. for a period comprised between 1 hour        and 10 hours with an air flow comprised between 0.1 m³/h and 1        m³/h.    -   the water-based mixture of aluminum hydroxide and the catalytic        precursors is dried by spray drying;    -   the dried mixture is calcinated at a temperature comprised        between 200 and 600° C. for a period comprised between 1 and 24        hours with an air flow comprised between 0.1 m³/h and 1 m³/h;    -   the calcinated product is ground to a powder with a volume        median particle diameter (D₅₀) of less than 450 μm;    -   the aluminum hydroxide is characterized by a specific surface        area (BET) comprised between 5 and 16 m²/g;    -   the aluminum hydroxide is selected from gibbsite or bayerite;    -   the cobalt-based precursor(s), the vanadium-based precursor(s),        the molybdenum-based precursor(s) and the support precursor have        a purity of at least 95%;    -   the cobalt-based precursor is cobalt(II) acetate tetrahydrate        and/or cobalt(II) nitrate tetrahydrate, the vanadium-based        precursor is ammonium metavanadate and the molybdenum-based        precursor is ammonium heptamolybdate tetrahydrate;    -   the polycarboxylic acid is a blend of citric acid and malic acid        wherein the mole ratio malic acid/citric acid is between 0.5 and        5.

The present disclosure further discloses a method for producingmulti-walled carbon nanotubes from the iron-free supported catalyst,comprising the steps of:

-   -   charging the catalyst into a reactor;    -   heating the catalyst to a temperature comprised between 500° C.        and 900° C.;    -   supplying a carbon source to the reactor while maintaining the        temperature comprised between 500° C. and 900° C.;    -   contacting the catalyst with the carbon source for a time period        of at least 1 minute.

Preferred embodiments of the method for the production of multi-walledcarbon nanotubes disclose one or more of the following features:

-   -   the space time between catalyst and carbon source is between 0.1        and 0.8 g·h/mole;

the carbon source is selected from the group consisting of methane,ethylene, acetylene, methanol, ethanol and mixtures thereof.

The present disclosure further discloses multi-walled carbon nanotubesobtained by the method for the production of multi-walled carbonnanotubes of the present disclosure, comprising between 0.1 and 13% byweight, preferably 1 and 10% by weight of the iron-free supportedcatalyst, said iron-free supported catalyst being obtained by the methodfor the preparation of iron-free supported catalyst of the presentdisclosure.

The present disclosure further discloses a polymer matrix comprisingsaid multi-walled carbon nanotubes, obtained by the method of thepresent disclosure.

The present disclosure further discloses the use of said multi-walledcarbon nanotubes, obtained by the method of the present disclosure, inbatteries.

DETAILED DESCRIPTION

The present disclosure discloses a supported iron-free catalyst givingrise to increased selectivity in multi-walled nanotube production withspecific characteristics, said improved multi-walled selectivity beingobtained at a high yield while catalyst consumption is reduced. Thepresent disclosure also discloses an economically attractive process forobtaining said supported catalyst.

By iron-free catalyst, the present disclosure means that the ironcontent is reduced as much as possible, with the exception ofunavoidable traces. Nevertheless the iron content within the overalltransition metal content is less than 1000 ppm, preferably less than 500ppm, more preferably less than 200 ppm, most preferably less than 100ppm.

In a first embodiment of the present disclosure, the supported catalystis an iron-free two-component catalyst comprising a first cobalt-basedcatalytic component and a second vanadium-based catalytic component,both preferably in the form of oxide, and supported on a supportcomprising aluminum oxide (Al₂O₃) and/or aluminum hydroxide (Al(OH)₃)and aluminum oxide hydroxide (AlO(OH)) (further called the “supportelement”)

In a second embodiment of the present disclosure, the supported catalystis an iron-free three-component graphitization catalyst comprising afirst cobalt-based catalytic component, a second vanadium-basedcatalytic component and a molybdenum-based catalytic component, allpreferably in the form of oxide, and supported on a support comprisingaluminum oxide and/or aluminum hydroxide and aluminum oxide hydroxide(further called the “support element”).

Preferably, the support precursor is aluminum hydroxide, more preferablygibbsite or bayerite.

Preferably, the support precursor is characterized by a volume medianparticle diameter (D₅₀) of less than 70 μm and a specific surface areaof less than 20 m²/g.

Preferably, the support precursor is gibbsite, characterized by aspecific surface area between 3 and 18 m²/g, preferably between 5 and 16m²/g.

Preferably, the cobalt-based catalytic precursor of the graphitizationcatalyst is obtained from a cobalt-based precursor, said precursor beinga cobalt salt, a cobalt oxide or a cobalt compound such asCo(NO₃)₂·6H₂O; Co₂(CO)₈ and Co(OAc)₂·4H₂O.

Preferably, the vanadium-based catalytic precursor of the graphitizationcatalyst is obtained from a vanadium-based precursor, said precursorbeing a vanadium salt, a vanadium oxide or a vanadium compound such asNH₄VO₃.

Preferably, the molybdenum-based catalytic precursor of thegraphitization catalyst is obtained from a molybdenum-based precursor,said precursor being a molybdenum salt, a molybdenum oxide or amolybdenum compound such as (NH₄)₆Mo₇O₂₄·4H₂O; Mo(CO)₆ or (NH₄)₂MoS₄.

The present disclosure also discloses a method for the production ofsaid supported catalyst comprising the steps of:

-   -   adding:        -   in the method of the first embodiment, a specific amount of            water comprising a specific amount of one or more            polycarboxylic acid(s) and/or the salts thereof to a            specific amount of one or more vanadium-based precursor(s)            and mixing until a transparent solution is obtained;        -   in the method of the second embodiment, a specific amount of            water comprising a specific amount of one or more            polycarboxylic acid(s) and/or the salts thereof to a            specific amount of one or more vanadium-based precursor(s)            and a specific amount of one or more molybdenum-based            precursor(s), and mixing until a transparent solution is            obtained;    -   contacting the one or more cobalt-based precursor(s) with the        aqueous solution comprising the vanadium-based precursor(s) and        the optional molybdenum-based precursor(s), wherein said one or        more cobalt-based precursors are added in the form of a powder        or a wetted powder or an aqueous solution or any form having a        water content comprised between powder and aqueous solution;    -   adding the support precursor and mixing during at least 1        minute;    -   drying the mixture by appropriated means, preferably at a fixed        predetermined temperature of at least 100° C. for at least 1        hours with an air flow of at least 0.1 m³/h;    -   calcinating the mixture by appropriated means, preferably at a        fixed predetermined temperature of at least 200° C. for at least        1 hour with an air flow of at least 0.1 m³/h;    -   grinding the calcinated product to a volume median particle        diameter (D₅₀) of less than 450 μm.

The polycarboxylic acids used in the method of the present disclosureare preferably selected from the group consisting of dicarboxylic acids,tricarboxylic acids, tetracarboxylic acids and mixtures thereof.Examples of such multicarboxylic acids include oxalic acid, succinicacid, tartaric acid, malic acid, fumaric acid, malic acid, itaconicacid, citraconic acid, mesaconic acid, citric acid,2-butene-1,2,3-tricarboxylic acid and 1,2,3,4-butanetetracarboxylicacid.

By salts of polycarboxylic acid, the present disclosure means thepolycarboxylic acid wherein at least one carboxylic acid group isconverted into an ammonium or alkali metal salt.

Preferably, the polycarboxylic acid is citric acid or malic acid;preferably the polycarboxylic acid salt is the ammonium salt.

Preferably, one or more polycarboxylic acid(s) and/or salt(s) thereofare added in such an amount that the resulting aqueous solutioncomprises between 0.5 and 25%, more preferably between 4 and 15% ofpolycarboxylic acid(s) and/or salt(s).

Preferably, the polycarboxylic acid used in the method of the presentdisclosure is a blend of citric acid and malic acid wherein the moleratio malic acid/citric acid is between 0.5 and 5, preferably between1.5 and 2.5.

In the method of the first embodiment, 1000 g of support precursor isadded to an aqueous mixture obtained from mixing an aqueous solutioncomprising 5 to 70 g of vanadium-based precursor in between 300 to 3000g of water and between 80 and 850 g of cobalt-based precursor as apowder or as an aqueous mixture comprising up to 3000 g of water.

In the method of the second embodiment, 1000 g of support precursor isadded to an aqueous mixture obtained from mixing an aqueous solutioncomprising 5 to 70 g of vanadium-based precursor and 1 to 15 gmolybdenum precursor in between 300 to 3000 g of water and between 80and 850 g of cobalt-based precursor as a powder or as an aqueous mixturecomprising up to 3000 g of water.

In the method according to the present disclosure:

-   -   water, at a temperature comprised between 20 and 90° C.,        preferably between 50 and 70° C., comprising one or more        polycarboxylic acid(s) and/or salt(s) of polycarboxylic acid(s),        is added to the vanadium-based precursor and the optional        molybdenum-based precursor and mixed, for example by means of a        paddle mixer, for a period comprised between 5 and 60 minutes,        preferably between 10 and 20 minutes;    -   the cobalt-based precursor, either as a powder, a wetted        precursor or as an aqueous solution, is added to the aqueous        solution comprising the vanadium-based precursor and the        optional molybdenum precursor; when added as a wetted precursor        or as an aqueous solution, water, at a temperature comprised        between 20 and 90° C., preferably between 50 and 70° C., is        added to the cobalt-based precursor and mixed for a period        comprised between 5 and 60 minutes, preferably between 10 and 20        minutes;    -   the support precursor is added and mixed (to avoid clumping) to        the aqueous solution comprising cobalt-based precursor,        vanadium-based precursor and the optional molybdenum precursor;    -   after completing of the support precursor addition, the        resulting paste is further mixed for a period comprised between        5 and 60 minutes, preferably for a period comprised between 10        and 20 minutes;    -   the paste is transferred to ceramic crucibles with a large        opening and heated:        -   as a first step to a temperature comprised between 100 and            150° C., preferably between 110 and 130° C. for a period            comprised between 60 and 600 minutes, preferably between 150            and 330 minutes, said temperature being obtained using a            heating gradient comprised between 1.0 and 5.0° C./min.; and            an air flow between 0.1 and 1.0 m³/h, preferably between 0.4            and 0.6 m³/h;        -    and subsequently        -   as a second step to a temperature comprised between 200 and            600° C., preferably between 220 and 550° C., more preferably            between 250 and 550° C. for a period comprised between 1            hour and 24 hour, preferably between 60 and 600 minutes,            more preferably between 150 and 330 minutes, said            temperature being obtained using a heating gradient            comprised between 1.0 and 5.0° C./min. and an air flow            between 0.1 and 1.0 m³/h, preferably between 0.4 and 0.6            m³/h;    -   the calcinated product is ground to a volume median particle        diameter (D₅₀) of less than 450 μm, preferably less than 250 μm.

After both heating cycles, the support precursor is converted into acalcinated product, i.e. the support, comprising one or more componentsselected from the group consisting of hydroxides, oxide hydroxides andoxides while the catalyst precursors are converted into oxides, whereinthe graphitization catalyst is preferably present as a mixed oxide.

The type of the heat source used in both heating cycles is not limitedand may be, for example, induction heating, radiant heating, laser, IR,microwave, plasma, UV or surface plasmon heating.

The inventors have observed that the BET of the support precursor,Al(OH)₃, is an important parameter for obtaining an iron-free supportedcatalyst enabling the production of MWCNT at a high carbon yield.

In the method according to the present disclosure the BET of the Al(OH)₃support precursor is comprised between 3 and 18 m²/g, preferably between5 and 16 m²/g.

The conversion of gibbsite to boehmite, studied by X-Ray Diffraction, isfor example described by A. M. d A. Cruz et al. in Applied Catalysis A:General 167 (1998), pp. 203-213.

The qualitative and quantitative analysis of aluminum oxide hydroxide(boehmite) in aluminum oxide (bauxite) by X-Ray Diffraction is forexample described by G. A. B. Soares et al. in Rev. Esc. Minas, 2014,vol.67, n.1, pp.41-46.

The inventors have experienced that the presence of aluminum oxidehydroxide in the iron-free supported catalyst, can be easily identifiedwith certainty by X-Ray Diffraction, yet this quantification is subjectto uncertainty and thus should be limited to an estimation of the weightpercentage of AlO(OH) on the total of Al₂O₃, Al(OH)₃ and AlO(OH).

That aside, the inventors have observed that aluminum oxide hydroxide ispresent in an amount of at least 30% by weight, preferably of at least40% by weight, more preferably of at least 50% by weight, mostpreferably of at least 60% by weight and even of at least 70% by weightof the total of Al₂O₃, Al(OH)₃ and AlO(OH).

In the method of the present disclosure, the first heating cycle,intended to dry the paste, may be replaced by alternative drying methodswell known in the art, or combinations thereof. Among these, flashdrying or spray drying are widely used.

A typical supported catalyst according to the present disclosure isrepresented by the formula (Co_(v)V_(w))O_(y)·(support)_(z) or(Co_(v)V_(w)Mo_(x))O_(y)·(support)_(z).

The iron-free two component graphitization catalyst is characterized inthat:

-   -   it comprises aluminum oxide hydroxide, preferably at least 30%        by weight of aluminum oxide hydroxide based on the total weight        of aluminum oxide, aluminum hydroxide and aluminum oxide        hydroxide;    -   the mass ratio of cobalt to aluminum is between 5.8 10⁻² and 5.8        10⁻¹, preferably between 1.2 10⁻¹ and 4.3 10⁻¹;    -   the mass ratio of vanadium to aluminum is between 5.8 10⁻³ and        8.7 10⁻², preferably between 1.2 10⁻² and 5.8 10⁻².

The iron-free two component graphitization catalyst is furthercharacterized in that the mass ratio of cobalt to vanadium is comprisedbetween 2 and 15, preferably between 3.0 and 11.

The iron-free three component graphitization catalyst is characterizedin that:

-   -   it comprises aluminum oxide hydroxide, preferably at least 30%        by weight of aluminum oxide hydroxide based on the total weight        of aluminum oxide, aluminum hydroxide and aluminum oxide        hydroxide;    -   the mass ratio of cobalt to aluminum is between 5.8 10⁻² and 5.8        10⁻¹, preferably between 1.2 10⁻¹ and 4.3 10⁻¹;    -   the mass ratio of vanadium to aluminum is between 5.8 10⁻³ and        8.7 10⁻², preferably between 1.2 10⁻² and 5.8 10⁻²; and    -   the mass ratio of molybdenum to aluminum is between 1.2 10⁻³ and        2.3 10⁻², preferably between 1.7 10⁻³ and 1.7 10⁻².

The iron-free three component graphitization catalyst is furthercharacterized in that the ratio of cobalt mass to the combined vanadiumand molybdenum mass is comprised between 2 and 15, preferably between3.0 and 11.

The iron-free supported catalysts of the present disclosure arecharacterized by a XRD pattern, recorded in the 2θ range of 10° to 80°,having a maximum diffraction peak, defined as “a”, at a 2θ angle of 35°to 38°, wherein

-   -   the ratio b/a of the intensity of a diffraction peak at a 2θ        angle of 17° to 22°, defined as “b”, over the intensity of the        maximum diffraction peak “a”, is in the range of 0.10 to 0.7,        preferably in the range of 0.12 to 0.7, more preferably in the        range of 0.14 to 0.7;    -   the ratio c/a of the intensity of a diffraction peak at a 2θ        angle of 63° to 67°, defined as “c”, over the intensity of the        maximum diffraction peak “a”, is in a range of 0.51 to 0.7; and    -   both criteria b/a (0.10 to 0.7) and c/a (0.51 to 0.7) are met.

For the preparation of MWCNT, the supported iron-free catalyst isbrought into contact with a carbon source in the gas phase.

The use of the supported catalyst allows for growth of the carbonnanotubes by chemical vapor synthesis through decomposition of thecarbon source, leading to the production of the carbon nanotubeaggregate.

According to the chemical vapor synthesis, the iron-free graphitizationcatalyst is charged into a reactor and the carbon source in the gasphase is then supplied to the reactor at ambient pressure and hightemperature to produce the carbon nanotube aggregate in which the carbonnanotubes are grown on the supported catalyst. As described above, thecarbon nanotubes are grown by thermal decomposition of a hydrocarbon ascarbon source. The thermally decomposed hydrocarbon is infiltrated andsaturated in the graphitization catalyst and carbon is deposited fromthe saturated graphitization catalyst to form hexagonal ring structures.

The chemical vapor synthesis can be performed in such a manner that thesupported catalyst is fed into a reactor and at least one carbon sourceselected from the group consisting of C₁-C₆ saturated hydrocarbons,C₁-C₆ unsaturated hydrocarbons, C₁-C₂ alcohols and mixture thereof,optionally together with a reducing gas (e.g., hydrogen) and a carriergas (e.g., nitrogen), is introduced into the reactor at a temperatureequal to or higher than the thermal decomposition temperature of thecarbon source in the gas phase to a temperature equal to or lower thanthe melting point of the graphitization catalyst, for example, at atemperature comprised between 500 and about 900° C., preferably between600 and 800° C., more preferably between 650 and 750° C. Carbonnanotubes may be grown for 1 minute to 5 hours, preferably 1 minute to30 minutes after the carbon source is introduced into the supportedcatalyst.

Preferably, the space time, defined as the weight of supported catalystin grams divided by the flow of reactant stream in mole/h, at standardtemperature and pressure conditions, is comprised between 0.1 and 0.8g·h/mole, preferably between 0.2 and 0.6 g·h/mole during a periodcomprised between 10 and 30 minutes, preferably between 15 and 25minutes.

The type of the heat source for the heat treatment in the method forpreparing the MWCNT is not limited and may be, for example, inductionheating, radiant heating, laser, IR, microwave, plasma, UV or surfaceplasmon heating.

Any carbon source that can supply carbon and can exist in the gas phaseat a temperature of 300° C. or more may be used without particularlimitation for the chemical vapor synthesis. The gas-phase carbonaceousmaterial may be any carbon-containing compound but is preferably acompound consisting of up to 6 carbon atoms, more preferably a compoundconsisting of up to 4 carbon atoms. Examples of such gas-phasecarbonaceous materials include, but are not limited to, carbon monoxide,methane, ethane, ethylene, methanol, ethanol, acetylene, propane,propylene, butane, butadiene, pentane, pentene, cyclopentadiene, hexane,cyclohexane, benzene, and toluene. These gas-phase carbonaceousmaterials may be used alone or as a mixture thereof. The mixed gas ofreducing gas (e.g. hydrogen) and carrier gas (e.g. nitrogen) transportsthe carbon source, prevents carbon nanotubes from burning at hightemperature, and assists in the decomposition of the carbon source.

The iron-free catalyst according to the present disclosure allows forthe production of MWCNT at a carbon yield comprised between 800 and2500% by weight, preferably between 1000 and 2400% by weight, morepreferably between 1100 and 2300% by weight.

The carbon yield, in % by weight, is defined as:

100(m _(tot) −m _(cat))/m _(cat)

wherein m_(tot) is the total weight of product after reaction andm_(cat) is the weight of the catalyst used for the reaction.

EXAMPLES

The following illustrative examples are merely meant to exemplify thepresent disclosure but they are not intended to limit or otherwisedefine the scope of the present disclosure.

Example 1 Synthesis of the Iron-Free Two Component GraphitizationCatalyst

5000 parts by weight of water, at 60° C., comprising 277 parts by weightof citric acid and 387 parts by weight of malic acid, were added to 333parts by weight of ammonium metavanadate, and mixed during 15 minutesusing a paddle mixer, resulting in a first aqueous solution.

Similarly, 5000 parts by weight of water, at 60° C., were added to 4109parts by weight of cobalt(II) acetate tetrahydrate and mixed during 15minutes using a paddle mixer, resulting in a second aqueous solution.

The second aqueous solution was added to the first aqueous solution andmixed during 15 minutes using a paddle mixer.

To the mixture of the first and the second aqueous solution, 13333 partsby weight of aluminum hydroxide (Apyral® 200 SM—Nabaltec), with specificsurface area (BET) of 15 m²/g, was added and mixed during 15 minutesusing a paddle mixer.

The paste thus obtained was then transferred to ceramic crucibles with alarge opening and subjected to a heating process, wherein the paste washeated to 120° C. with a heating gradient of 2° C./min and an air flowof 0.5 m³/h and maintained at 120° C. for 5 hours.

After 5 hours at 120° C., the paste was further heated to a temperatureof 400° C. with a heating gradient of 2° C./min. and maintained at 400°C. for 5 hours while maintaining an air flow of 0.5 m³/h.

The solid material thus obtained was cooled down to room temperature andground, by means of a conical grinder, to a powder characterized by avolume median particle diameter (D₅₀) of 120 μm.

Example 2 Synthesis of the Iron-Free Three Component GraphitizationCatalyst

Example 1 was repeated with the exception that 5000 parts by weight ofwater, at 60° C., comprising 277 parts by weight of citric acid and 387parts by weight of malic acid, were added to 340 parts by weight ofammonium metavanadate and 64 parts by weight of ammonium heptamolybdatetetrahydrate, resulting in a first aqueous solution. The second aqueoussolution is obtained by adding 5000 parts by weight of water, at 60° C.,to 4931 parts by weight of cobalt(II) acetate tetrahydrate

To the mixture of the first and the second aqueous solution, 13333 partsby weight of aluminum hydroxide (Apyral® 200 SM—Nabaltec), with aspecific surface area (BET) of 15 m²/g, was added and mixed during 15minutes using a paddle mixer.

Example 3 to 8

In the examples 3 to 8:

-   -   the vanadium-based precursor is ammonium metavanadate;    -   the molybdenum-based precursor is ammonium heptamolybdate        tetrahydrate;    -   the cobalt-based precursor is cobalt(II) acetate tetrahydrate        for example 3 and examples 5 to 8;    -   the cobalt-based precursor is cobalt(II) nitrate tetrahydrate        for example 4    -   Al(OH)³ of example 3 is ALOLT 59 AF (Inotal) characterized by a        BET of 5.4 m²/g    -   Al(OH)₃ of example 4 is Hydral 710 (Huber) characterized by a        BET of 4 m²/g;    -   Al(OH)₃ of examples 5 and 6 is Apyral 40 CD (Nabaltec)        characterized by a BET of 3.5 m²/g;    -   Al(OH)₃ of examples 7 and 8 is Martinal OL-111 LE (Huber)        characterized by a BET of 10-12 m²/g.

Examples 3 to 7 are prepared using the process conditions of example 1,i.e. temperature and time period of mixing, drying and calcinatingconditions (temperature, heating gradient, time, air flow), and grindingconditions for obtaining a D₅₀ of about 120 μm, with the exception thatthe cobalt-based precursor is added as a powder to the aqueous solutioncomprising the vanadium-based precursor and the optionalmolybdenum-based precursor, said aqueous solution comprising 5000 partsby weight of water.

Example 8 is a comparative example wherein the support precursor iscalcinated before being added to, and mixed with, the aqueous mixturecomprising the cobalt-based precursor, the vanadium-based precursor andthe molybdenum-based precursor. The support precursor is firstimpregnated with water and stirred for 12 hours at 60° C., before beingdried at 60° C. and 100 mbar. Subsequently the dried support precursoris calcinated at a temperature of 400° C. during 5 hours under anitrogen atmosphere, whereupon the calcinated support is added to theaqueous mixture of catalyst precursors. The aqueous mixture comprisingthe cobalt-based precursor, the vanadium-based precursor and themolybdenum-based precursor is obtained from adding the cobalt-basedprecursor, as a powder, to the aqueous solution comprising thevanadium-based precursor, the molybdenum-based precursor and 5000 partsby weight of water. The resulting paste was heated to 120° C. with aheating gradient of 2° C./min and an air flow of 0.5 m³/h and maintainedat 120° C. for 5 hours.

Subsequently the paste was further heated to a temperature of 400° C.with a heating gradient of 2° C./min and maintained at 400° C. for 5hours while maintaining an air flow of 0.5 m³/h. Diffraction peakscorresponding to boehmite, AlO(OH), were not detected.

In table 1 the quantities of catalyst precursors, the support precursorand the polycarboxylic acid(s) and/or the salts thereof, in parts for5000 parts by weight of water, are reported for examples 3 to 8.

TABLE 1 Support Cobalt Vanadium Molybdenum Citric Malic Dibasic saltExample precursor precursor precursor precursor acid acid citric acid 33335 835 35 85 35 4 3335 970 100 5 50 85 5 6667 2667 267 33 200 296 66250 1200 70 60 135 120 7 6665 1810 225 55 140 195 8 2500 1430 65 10 105

Synthesis of MWCNT

1.0 g of the iron-free supported graphitization catalyst of examples 1to 8 were spread in a quartz vessel which subsequently was brought inthe center of a quartz tube-type reactor with an inlet and an outlet.

The center of the quartz tube reactor where the vessel comprising thecatalyst is located was heated to a temperature of 700° C.

Subsequently ethylene gas, nitrogen and hydrogen were allowed to flowthrough the quartz tube reactor at a flow rate of 1.744 l/min (C₂H₄);0.857 l/min (N₂) and 0.286 l/min (H₂) during 20 minutes.

In table 2, carbon yield (column 8) is given for the MWCNT (examples Ato H) (column 1) prepared using the catalysts of examples 1 to 8 (column2).

Furthermore table 2 shows

-   -   the ratio of cobalt to aluminum of the supported catalyst        (column 3);    -   the ratio of vanadium to aluminum of the supported catalyst        (column 4);    -   the ratio of molybdenum to aluminum of the supported catalyst        (column 5);    -   the ratio of cobalt to vanadium for the iron-free two component        graphitization supported catalyst and the ratio of cobalt to        vanadium and molybdenum for the iron-free three component        graphitization supported catalyst (column 6).    -   the BET (m²/g) of the respective Al(OH)₃ support precursors        (column 7).

TABLE 2 Iron-free supported catalyst MWCNT Example Example Co/V + BETAl(OH)₃ Yield MWCNT Catalyst Co/Al V/Al Mo/Al (Mo) (m²/g) (%) A 1 2.1210⁻¹ 3.11 10⁻² 6.7 15 1953 B 2 2.53 10⁻¹ 3.20 10⁻² 7.54 10⁻³ 6.4 15 2076C 3 1.71 10⁻¹ 1.26 10⁻² 13.6 5.4 1161 D 4 1.71 10⁻¹ 3.78 10⁻² 2.36 10⁻³4.3 4 891 E 5 2.71 10⁻¹ 5.07 10⁻² 7.78 10⁻³ 4.7 3.5 891 F 6 1.31 10⁻¹1.36 10⁻² 1.50 10⁻³ 4.6 3.5 919 G 7 1.85 10⁻¹ 4.25 10⁻² 1.25 10⁻² 3.410-12 1862 H 8 3.91 10⁻¹ 3.16 10⁻² 5.49 10⁻³ 10.6 10-12 554

As clearly appears from table 2, the iron-free supported catalystsaccording to the present disclosure (examples 1 to 7) give rise to aMWCNT (examples A to G) with a carbon yield of at least 800%, contraryto the MWCNT obtained from a process using an iron-free supportedcatalyst (example 8), wherein the support precursor is calcinated beforeimpregnation with the catalyst precursors. MWCNT with the highest carbonyield are obtained from iron-free supported catalysts, prepared fromAl(OH)₃ support precursor, characterized by a BET comprised between 10and 15 m²/g. The iron-free supported catalyst of example 8 (=comparativeexample) gives rise to a MWCNT (example H) with a carbon yield of 554%,though said supported catalyst is prepared from Al(OH)₃ supportprecursor with a BET of 10-12 m²/g. For the iron-free supported catalystof example 8 (=comparative example), diffraction peaks corresponding toboehmite, AlO(OH), were not detected.

The inventors have surprisingly observed that a calcination temperature,of the dried mixture of aluminum hydroxide and the catalytic precursors,comprised between 200° C. and 600° C. results in multi-wall carbonnanotubes with a high carbon yield, contrary to multi-wall carbonnanotubes resulting from a supported catalysts obtained from the samedried mixture but calcinated at a temperature above 600° C.

The inventors have observed as well that also the drying method has aninfluence, although to a lesser extent, on carbon yield of the finalmulti-wall carbon nanotubes.

The influence of the calcination temperature is reflected by the ratiosof the intensity of diffraction peaks in the XRD pattern of thesupported catalyst, recorded in the 2θ range of 10° to 80°.

In the XRD pattern, a diffraction peak with maximum intensity at a 2θangle of 35° to 38° is defined as “a”. When the intensity of thediffraction peak at a 2θ angle of 17° to 22° is defined as “b” and theintensity of the diffraction peak at a 2θ angle of 63° to 67° is definedas “c”, multi-wall carbon nanotubes with high carbon yield are preparedwhen using the iron-free supported catalyst where both conditions ofintensity ratios (b/a and c/a), being the ratio b/a comprised between0.10 and 0.7 and the ratio c/a comprised between 0.51 and 0.7, are met.

In table 3, the value of the 2θ angle, the net intensity at said 2θangle and the intensity ratios b/a and c/a of the supported catalyst,obtained from different drying methods and calcination temperatures, arereported.

In table 4, the carbon yield, in % by weight of MWCNT of example B,obtained from the iron-free supported catalyst of example 2 is reportedfor drying the catalyst precursor:

-   -   during 5 hours at 120° C., wherein the precursor paste was        heated to 120° C. with a heating gradient of 2° C./min and an        air flow of 0.5 m³/h;    -   diluting the precursor paste, so that 10,000 parts of precursor        paste are converted in 25,000 parts of precursor dispersion,        sufficiently fluid for peristaltic pumping to the spray dryer        equipment GB-210A from Yamato Scientific with settings:        -   blower: 0.5 m³/h (=hot air flow for drying))        -   atomizer: 0.1 MPa (=air pressure generating the spray)        -   drying temperature: 150° C. (=air temperature at the            entrance of the drying column)        -   pump: 7 (=flow rate of the pumped liquid, depending on the            speed of the pump and the viscosity of the liquid, and            therefore on its dilution. In the present experiment the            flow rate equals +/−at 17 g/min.)

The inventors have observed that calcination of the dried mixture ofaluminum hydroxide and the catalytic precursors, at a temperature of700° C., results in multi-wall carbon nanotubes with lower carbon yield;at a calcination temperature of 700° C., the intensity ratio (b/a) isnot met. Diffraction peaks corresponding to boehmite (AlO(OH)) are notdetected.

A reduced carbon yield, in % by weight, relative to the carbon yield, in% by weight, of the MWCNT of example B (Carbon Yield=2076%), wasobtained when repeating example B using the iron-free three componentgraphitization catalyst of example 2, but calcinated for 5 hours at 550°C. and 700° C. respectively. As such, a reduction of about 14% carbonyield, relative to carbon yield of example B, was observed for thecatalyst of example 2, but calcinated for 5 hours at 550° C. (CarbonYield=1781%), while a reduction of 42% carbon yield, relative to carbonyield of example B, was observed for the catalyst of example 2 butcalcinated for 5 hours at 700° C. (Carbon Yield=1211%),

The spray dried iron-free three component graphitization catalyst ofexample 2, calcinated during 1 hour at 600° C., resulted in MWCNT at acarbon yield of 1840%.

TABLE 3 a (35-38°) b (17-21°) c (63-67°) calcination calcination net netnet temperature time drying angle intensity angle intensity b/a angleintensity c/a 250° C. 5 h furnace 36.730° 4246 20.325° 2836 0.66866.393° 2641 0.622 400° C. 5 h furnace 36.947° 3665 19.543° 831 0.22765.368° 2528 0.69 550° C. 5 h furnace 36.867° 6767 19.048° 1016 0.1565.278° 3510 0.519 700° C. 5 h furnace 36.913° 8639 19.215° 621 0.07265.544° 5169 0.598

TABLE 4 200° C. 250° C. 400° C. 550° C. 700° C. Furnace Spray FurnaceSpray Furnace Spray Furnace Spray Furnace Spray drying drying dryingdrying drying drying drying drying drying drying 1 h 1846% 1755% 1864%2035% 2227% 2216% 2079% 2084% 1203% 900% 5 h 1833% 1779% 2130% 2047%2076% 2249% 1781% 1887% 1211% 652% 24 h  1852% 1763% 2158% 2102% 2147%2172% 1844% 1975%  652% 627% Removed Shading

1. An iron-free supported catalyst for the selective conversion ofhydrocarbons to carbon nanotubes, said catalyst comprising cobalt andvanadium as active catalytic metals in any oxidation state on a catalystsupport comprising at least 30% by weight of aluminum oxide hydroxidebased on the total of aluminum hydroxides and/or aluminum oxides andaluminum oxide hydroxide, as determined by X-Ray diffraction, wherein:the mass ratio of cobalt to vanadium is between 2 and 15; the mass ratioof cobalt to aluminum is between 5.8 10⁻² and 5.8 10⁻¹; and the massratio of vanadium to aluminum is between 5.8 10⁻³ and 8.7 10⁻².
 2. Theiron-free supported catalyst according to claim 1, wherein: the massratio of cobalt to vanadium is between 3.0 and 11; the mass ratio ofcobalt to aluminum is between 1.2 10⁻¹ and 4.3 10⁻¹; and the mass ratioof vanadium to aluminum is between 1.2 10⁻² and 5.8 10⁻².
 3. Theiron-free catalyst according to claim 1 comprising molybdenum asadditional active catalyst, wherein: the mass ratio of molybdenum toaluminum is between 1.2 10⁻³ and 2.3 10⁻²; and the mass ratio of cobaltto the combined mass of vanadium and molybdenum is between 2 and
 15. 4.The iron-free catalyst according to claim 3, wherein: the mass ratio ofmolybdenum to aluminum is between 1.7 10⁻³ and 1.7 10⁻²; and the massratio of cobalt to the combined mass of vanadium and molybdenum isbetween 3 and
 11. 5. The iron-free catalyst according to claim 1 havinga maximum diffraction peak at a 2θ angle of 35° to 38° in the XRDpattern recorded in the 2θ range of 10° to 80°, wherein when theintensity of the maximum diffraction peak and the intensity of adiffraction peak at a 2θ angle of 17° to 22° are defined as “a” and “b”,respectively, the ratio b/a is 0.10 to 0.7, and when the intensity of adiffraction peak at a 2θ angle of 63° to 67° is defined as “c”, theratio c/a is in a range of 0.51 to 0.7.
 6. The iron-free catalystaccording to claim 1, wherein the catalyst support comprises at least40% by weight of aluminum oxide hydroxide based on the total of aluminumhydroxides and/or aluminum oxides and aluminum oxide hydroxide, asdetermined by X-Ray diffraction.
 7. A method for the production of theiron-free supported catalyst of claim 1 comprising the steps of:contacting an aqueous solution comprising one or more polycarboxylicacid(s) and/or the salt(s) of polycarboxylic acids, with one or morevanadium-based precursor(s) and optionally with one or moremolybdenum-based precursor(s); contacting the one or more cobalt-basedprecursor(s) with the aqueous solution comprising the vanadium-basedprecursor(s) and the optional additional molybdenum-based precursor(s)to form a water-based mixture of catalytic precursors; contactingaluminum hydroxide, having a BET comprised between 3 and 18 m²/g, withthe water based mixture comprising the catalytic precursors to form awater-based mixture of aluminum hydroxide and the catalytic precursors;drying the water-based mixture of aluminum hydroxide and the catalyticprecursors to form a dried mixture; calcinating the dried mixture at atemperature comprised between 200° C. and 600° C. to form a calcinatedproduct comprising at least 30% by weight of aluminum oxide hydroxidebased on the total of aluminum hydroxides and/or aluminum oxides andaluminum oxide hydroxide, as determined by X-Ray diffraction; andgrinding the calcinated product to a powder.
 8. The method according toclaim 7, wherein the water-based mixture of aluminum hydroxide and thecatalytic precursors is dried at a predetermined temperature of at least100° C. for at least 1 hour with an air flow of at least 0.1 m³/h. 9.The method according to claim 7, wherein the water-based mixture ofaluminum hydroxide and the catalytic precursors is dried at apredetermined comprised between 100 and 150° C. for a period comprisedbetween 1 hour and 10 hours with an air flow comprised between 0.1 m³/hand 1 m³/h.
 10. The method according to claim 7, wherein the water-basedmixture of aluminum hydroxide and the catalytic precursors is dried byspray drying.
 11. The method according to claim 7, wherein the driedmixture is calcinated at a temperature comprised between 220 and 550° C.for a period comprised between 1 and 24 hours with an air flow comprisedbetween 0.1 m³/h and 1 m³/h.
 12. The method according to claim 7,wherein the calcinated product is ground to a powder with a volumemedian particle diameter (D₅₀), of less than 450 μm.
 13. The methodaccording to claim 7, wherein aluminum hydroxide is characterized by aspecific surface area (BET) comprised between 5 and 16 m²/g.
 14. Themethod according to claim 7, wherein the aluminum hydroxide is selectedfrom gibbsite or bayerite.
 15. The method according to claim 7, whereinthe cobalt-based precursor(s), the vanadium-based precursor(s), themolybdenum-based precursor(s), and the support precursor have a purityof at least 95%.
 16. The method according to claim 7, wherein thecobalt-based precursor is cobalt(II) acetate tetrahydrate and/orcobalt(II) nitrate tetrahydrate, the vanadium-based precursor isammonium metavanadate, and the molybdenum-based precursor is ammoniumheptamolybdate tetrahydrate.
 17. The method according to claim 7,wherein the polycarboxylic acid is a blend of citric acid and malic acidwherein the mole ratio malic acid/citric acid is between 0.5 and
 5. 18.The method of claim 7, further comprising producing multi-walled carbonnanotubes by: charging the catalyst into a reactor; heating the catalystto a temperature comprised between 500° C. and 900° C.; supplying acarbon source to the reactor while maintaining the temperature comprisedbetween 500° C. and 900° C.; and contacting the catalyst with the carbonsource for a time period of at least 1 minute.
 19. The method accordingto claim 18, wherein the space time between catalyst and carbon sourceis between 0.1 and 0.8 g·h/mole, wherein the space time is defined asthe weight of supported catalyst in grams divided by the flow ofreactant stream in mole/h, at standard temperature and pressureconditions.
 20. The method according to claim 18, wherein the carbonsource is selected from the group consisting of methane, ethylene,acetylene, methanol, ethanol, and mixtures of methane, ethylene,acetylene, methanol, and/or ethanol.
 21. Multi-walled carbon nanotubes,obtained by the method according to claim 18, comprising between 0.1 and13% by weight of the catalyst.
 22. A polymer matrix comprising themulti-walled carbon nanotubes according to claim
 21. 23. Use of themulti-walled carbon nanotubes according to claim 21 in batteries.